In many cases, an automatic transmission is arranged between the output shaft of the internal combustion engine and an axle shaft of a vehicle. An ECU (electronic control unit) automatically controls the transmission gear ratio of the automatic transmission according to a vehicle running state such as an operation amount of an accelerator and a vehicle speed.
For example, in some cases, a downshift switch and an upshift switch are provided on the steering wheel to allow the driver to change the transmission gear ratio manually.
When the engine speed is great and the driver manually operates to downshift, or increases the transmission gear ratio, the engine speed may be excessively high (over revolution). That is, the engine can over-rev.
In a controller disclosed in Patent Document 1, when the engine speed is greater than a predetermined upper limit speed, change of the transmission gear ratio is inhibited. That is, the controller does not downshift even if the driver manually operates to downshift. Accordingly, an engine overspeed is avoided.
Generally, a little time difference occurs between the time when the driver manually operates to request to change the transmission gear ratio and the time when the transmission gear ratio is actually changed. Even during such a transmission gear ratio changing period, the engine speed change from moment to moment. That is, during a transmission gear ratio changing period, the changing state of the engine speed is not constant. Therefore, to prevent an engine overspeed in every case, it is necessary to set the upper limit speed to be small. That is, the engine upper limit speed is set to be small to allow a margin.
However, setting the engine upper limit speed to be small to allow a margin reduces the vehicle operational region in which downshift is allowed, which lowers the vehicle maneuverability.
Patent Document 1: Japanese Laid-Open Patent Publication No. 10-89466